Gathered together or assembled, often with effort; summoned up courage or energy.
From Old French 'monstrer' meaning 'to show,' from Latin 'monstrare.' The military sense developed because armies were 'shown' or paraded for inspection, and eventually 'muster' meant to gather the soldiers themselves.
Military heritage lives in this word—a 'muster roll' was literally the list of soldiers presented for inspection. When you 'muster courage,' you're still using the military metaphor of gathering your inner troops to face something difficult.
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